This invention relates to a pipeline monitoring system and, more particularly, to pipeline monitoring systems including a processing device designed to derive an estimation of deposit deposition along the pipeline.
In producing hydrocarbons from an underwater well or bore hole, it is usual to place a xe2x80x9cchristmas treexe2x80x9d, which contains a number of flow control valves, on the sea or lake-bed at the top of the well or bore hole, and to connect the lower end of a production pipeline to that christmas tree and to connect the upper end of the production pipeline, via a riser, to a ship, rig or shore based hydrocarbon collection facility. Production of oil from such wells introduces considerable engineering problems because the cooling that occurs as oil travels through the pipeline along the sea or lake bed may cause scales, asphaltenes, waxes and hydrates to come out of solution and form solid particles. These particles may form solid deposits on the inner walls of the pipeline, or agglomerate to form slurries or plugs. As amounts of deposits, plugs and/or slurries within the pipeline increases, the operational effectiveness of the pipeline is reduced. If the deposits were allowed to build up over time, oil flow through the pipeline would become progressively more restricted until insufficient energy existed within the reservoir or pipeline system to economically extract the oil.
To reduce the rate of cooling and hence the rate of particle formation, it is common to thermally insulate the pipeline by jacking it with a thermally insulating material. It is known also to contain the pipeline in a bundle with pipelines along which hot fluids, such as hot water or reservoir fluids from a well or wells at a higher temperature, relying on thermal conduction from the hot fluid pipelines to heat the cooler pipeline or pipelines. This bundle will usually also be thermally insulated. Techniques used to insulate such bundles can involve the use of air or nitrogen, which may be trapped in a rock wool or glass beads which are packed in the spaces between the pipelines and an outer carrier pipe in which the pipelines are contained. Alternatively a vacuum may be maintained in such an outer carrier pipe. With these schemes, the conduction of heat from the pipeline to the surrounding body of water is reduced, providing generally greater oil temperatures, with particle formation and hence deposition rates being accordingly reduced.
To reduce deposition rates further, deposit inhibitor chemicals often are continually or periodically injected into the pipeline at the well head, by way of the christmas tree, or downstream of the well-read by way of a manifold arrangement. The cost of the infrastructure required for chemical injection and the operational costs of providing the necessary chemicals are each considerable costs.
Nevertheless, deposits will build up which, if a pipeline is to he maintained in use will need to be removed. A technique commonly used in such deposit removal involves the periodic insertion of mechanical pigs into the lower end of the pipeline, for later removal at the upper end. This technique is used both to remove any deposit formation that may have built up and as a preventative measure before signification deposits appear although it can be disruptive and/or expensive.
As the increased resistance to flow offered by deposits ill a pipeline can be very costly in terms of production rates obtainable, it would seem to be best to carry out mechanical pigging and/or chemical solvent injection at intervals calculated to be sufficient to prevent blockage or severe flow restriction being caused by deposit formation. However, because the condition of tie deposits within the pipeline is so difficult to ascertain, deposit removal is usually carried out more frequently than the calculated frequency, as the cost of pipeline replacement is far greater than the costs involved with deposit removal. This procedure reduces the probability of blockage or severe flow restriction occurring to and acceptable level.
Therefore, it is desirable to monitor the formation of deposits in the pipeline. U.S. Pat. No. 3,913,378 proposes monitoring deposit formation by measuring temperature differential across the wall of a pipe, and across the interface between the flowing fluid and the pipe. However, this system incorporates thermocouples embedded in the wall of the pipe and inserted in the bore of the pipe. This is impractical for the purpose of monitoring pipelines of hydrocarbon wells.
In accordance with a first aspect of this invention, a system for monitoring a pipeline comprises a processing device arranged to receive temperature measurement signals from a distributed temperature measurement means associated including at least one optical fibre with the pipeline, and being designed to derive from those signals an estimation of deposit deposition at at least one position along the pipeline.
The processing device may contain a model of at least deposit deposition on inner walls of the pipeline.
The model preferably relates to deposit type and to deposit thickness.
The model may comprises a series of equations, which may be polynomial equations, or may be a look-up table or a number of associated look-up tables, stored or otherwise carried within the processor means. The processor means may be a general purpose computer, a computer system used also for other tasks, application specific computer hardware including a processor, or any other type of processor means.
The pipeline may be an underwater hydrocarbon pipeline, a cold climate pipeline such as an arctic oil pipeline, or maybe any other type of pipeline used to carry fluid at a temperature greater than the ambient temperature where cooling of the fluid may be detrimental to its transportation or its value.
The term deposit thickness will be understood to refer to deposits which have ambiguous boundaries with fluid flowing in the pipeline as well as those which have clear and defined boundaries with the fluid. In the former case, the deposit deposition measurement signals may communicate an approximation of thickness which corresponds to a boundary which an operator dictates is of interest, such as the boundary, between deposits having less than and deposits having greater than a particular viscosity. In some cases, it may be preferred to treat the fluid and the deposits as the same class of material and determine boundaries only on the basis of such characteristics as viscosity.
The term distributed temperature measurement means will be understood to include, in addition to apparatus which can measure temperature in a truly distributed sense, apparatus which can measure temperature at points along the pipeline which are a small distance apart. These points may he although are not necessarily, between 1 m and 100 m apart. The more important requirements of the temperature measurement means may be accuracy of measurement and, particularly, coverage with respect to the length of the pipeline rather than resolution of temperature measurement along (a small length of the pipeline.
The temperature measurement means preferably includes one or more optical fibres associated with the pipeline, in which case the or each optical fibre may be sheathed in a stainless steel tube. The or at least one optical fiber may be located within the pipeline, in which case the optical fiber or fibers may be attached to the inner wall of the pipeline periodically along its or their length.
Alternatively, the or at least one optical fibre is located adjacent the outside wall of a pipe forming part of the pipeline, in which case the optical fibre or fibres are preferably located between the pipe and a corrosion preventing layer.
The temperature measurement means may further include, advantageously, means to detect characteristics of a laser light pulse back scatted by the or each fibre, and means to provide therefrom said temperature measurement signals.
In accordance with a preferred feature, the processing device is also arranged to receive and designed to derive an estimation of deposit deposition from pressure measurement signals provided by a pressure measurement means arranged to determine fluid pressure at at least one location within the pipeline.
In accordance with another preferred feature, the processing device is also arranged to receive and designed to drive an estimation of deposit deposition from deposit thickness measurement signals provided by a thickness measurement means including at least one measurement transducer located at a discrete position along the pipeline.
Here, the measurement transducer may be associated with a measurement device, and the thickness measurement means may including at least two such measurement devices. In this case, tile thickness measurement meals may comprise between two and ten thickness measurement devices located at discrete positions along the pipeline, which is preferably three, four or five thickness measurement devices. For optimum utility, the or at least one measurement transducer is located at a position along the pipeline near which deposits are expected to form during use.
In accordance with a preferred aspect of the invention, the or each measurement transducer is an ultrasonic transducer arranged to transmit ultrasonic pulses into tile pipeline. Here, the thickness measurement means may be operable to detect a distance between a boundary between the pipe and a deposit and a boundary between the deposit and a hydrocarbon flowing along tile pipeline at the or each discrete position. In this case, the thickness measurement means may he operable also to detect a distance between the boundary of the pipe and the deposit and a boundary between two different types of deposit.
In such ultrasonic systems, a determined fraction of an acoustic signal reflected by a deposit may be used to determine the deposit type.
In accordance with a second aspect of this invention, apparatus for obtaining an indication of the nature of deposits along a pipeline comprises means for measuring the thickness of such deposits at at least one discrete point along the pipeline; means for measuring a pipeline which means incoluding at least one optical fibre and means for using the two measurements to estimate the thickness of the deposits at different positions along the pipeline.